There are a lot of words in the Japanese language, and WK only prioritizes ones that will help you learn the kanji they teach, even if you’ll rarely (if ever) see them in the wild. There are also a lot that are kana-only, which WK doesn’t teach.
There are lots of different ways that people learn vocab off WK. Many swear by Anki—there are a lot of premade decks, including most-frequent words and kana-only words; and/or they’ll make their own decks from words they come across in the wild and want to learn. I personally found Anki very user-unfriendly though so it doesn’t work for me, but it does seem to work for a lot of people.
I use KameSame, which is similar to WK but tests you on production + recall, rather than pronunciation + meaning. It connects to WK and you can get extra practice on words and kanji you’ve learned on WK after they’ve reached a certain stage if you so desire (default is Guru, I have it at Burned), and it has several vocab lists plus a search feature that lets you add words manually.
BunPro, if you use (or later start using) that for grammar, has recently started adding vocab as well, and it includes a lot that’s not on WK, including kana-only vocab. That also connects to WK, and you can choose to sync your WK vocab with it so you don’t have to go through it again.
Some people will just learn vocab by reading and looking up unknown words as they go without making SRS flashcards for them, whether that means they bookmark them in the dictionary app they use, write them down on paper, make physical flashcards, etc. (I’m also in this camp. SRS might be more efficient, but it’s also easy to get burned out, so there are periods where I’ll slow way down on my SRSs and not add anything except maybe 5-10 items on WK if I think I can handle it.)
It all depends on your learning style and what works best for you (and how much you can handle—you’ll have to be careful not to take on too much and burn yourself out), but you will absolutely have to learn vocab outside of WK at some point, regardless of how you go about it.